In recent years, a display device provided with a touch sensor has attracted attention. The display device provided with a touch sensor is called a touch panel, a touch screen, or the like (hereinafter referred to simply as a touch panel). The touch sensor is classified by principle of operation under resistive touch sensor, capacitive touch sensor, optical touch sensor, and the like. In any type of touch sensor, data can be input when an object is in contact with or adjacent to a display device.
When a sensor which detects light is provided as an optical touch sensor (also referred to as a photo sensor) in the touch panel, a display screen also serves as an input region. One example of a device including such an optical touch sensor is a display device having an image capturing function, which is enabled by providing a contact area sensor which captures an image (see, for example, Patent Document 1). As for a touch panel including an optical touch sensor, light is emitted from a touch panel. When an object exists at a predetermined position of the touch panel, light at the region where the object exists is blocked by the object, and part of the light is reflected. A photo sensor (also referred to as a photoelectric conversion element) which can detect light is provided in a pixel of the touch panel, and by detecting the reflected light, the photo sensor recognizes the existence of the object in the region where the light is detected.
In addition, it has been attempted to give a personal identification function etc. to an electronic device typified by a portable information terminal such as a mobile phone and the like (for example, see Patent Document 2). Identifications through a finger print, a face, a hand print, a palm print, a pattern of hand veins, and the like are used for personal identification. When the personal identification function is provided into a portion other than the display portion, the number of components increases, and the weight or price of the electronic device could increase.
In addition, in a touch sensor system, a technique to select an image processing mode to detect the position of a fingertip depending on the brightness of outside light is known (for example, see Patent Document 3).